Dual frequency horn antenna



June 1957 J s. AJIOKA ETAL DUAL FREQUNCY HORN ANTENNA 2 Sheets-Sheet 1 N EN Filed June 1,

H a mg M WW7 A 4 Z w q MN I Q Tl IWI u hw United States Patent 3,325,817 DUAL FREQUENCY HORN ANTENNA James S. Ajiolsa, Fullerton, and George I. Tsuda, Garden Grove, Calif assignors to Hughes Aircraft Company, Culver City, Calif, a corporation of Delaware Filed June 1, 1964, Ser. No. 371,562 7 Claims. (Cl. 343779) This invention relates to antenna systems and particularly to a compact and simple antenna feed that provides a high degree of isolation or filtering between the transmit and receive signals.

Communication antenna feeds which are utilized for transmitting signals at a first frequency and receiving signals at a second frequency require isolation between transmitted energy being coupled into the received energy when the first and second frequencies are respectively high and low frequencies, for example. conventionally, relatively large and complex stop band filters or highly lossy filters are utilized in the received signal path subsequent to the feed aperture to provide filtering at microwave frequencies. A feed and reflector system that would provide a high degree of filtering or isolation between the transmit and receive signals without a complex or lossy filter arrangement while maintaining a desirable antenna gain characteristics, would be highly advantageous to the art.

It is therefore an object of this invention to provide an antenna feed system having an inner and outer horn and providing a high degree of isolation or filtering between the two horns.

It is a further object of this invention to provide an antenna system that has the transmit and receive feed horns arranged to provide a high degree of isolation between the transmit and receive energy while maintaining desired pattern characteristics.

It is a still further object of the invention to provide an antenna feed system utilizing pyramidal horns that transmit energy at one frequency and receives energy at a relatively lower frequency and that substantially eliminates the high frequency energy leakage into the receive aperture.

It is another object of this invention to provide an antenna feed that isolates the transmit and receive apertures substantially without defocus of the energy pattern so as to maintain desirable secondary antenna pattern characteristics.

It is still another object of this invention to provide a compact antenna feed system for transmitting energy at a relatively high frequency and receiving energy at a relatively low frequency that overcomes undesired higher modes in the received energy.

The antenna feed system in accordance with the principles of the invention, utilizes a first or transmit horn positioned inside of a second or receive horn both having throat dimensions selected in accordance with the respective high and low signal frequencies. The aperture of the transmit horn is projected in front of the aperture of the receive horn a distance selected to provide a desired degree of isolation or filtering. It has been found that the flare angles of the transmit and receive horns may be selected so that the phase centers of the transmit and receive signals are substantially coincident with each other and may be coincident with the focal point of a parabolic antenna. Any difference in coincidence of the two phase centers, which may result from design compromises or from limited time spent in experimental design of parameters, is selected to be substantially equal portions of the respective wavelengths. Thus, the system provides isolation between the transmit and receive signals substantially without defocus of the primary pattern and without the consequent decreasing of side lobe energy in the secondary pattern in space. Because the internal transmit horn prevents tapering the receive horn to dimensions which eliminate undesired modes, a balanced feed is utilized for transferring the receive energy to the utilization system.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the accompanying description taken in connection with the accompanying drawings, in which like reference characters refer to like parts, and in which:

FIG. 1 is a top view of the feed system in accordance with the invention shown in the plane of the electrical vectors and with portions partially broken away for ease of understanding;

FIG. 2 is a section taken at line 2-2 of FIG. 1 for explaining the arrangement of the feed system in the plane of the magnetic vectors;

FIG. 3 is a diagram showing the parabolic reflecting surface and the antenna lobe primary patterns for explaining the selection of horn flare angles to provide the substantially coincident phase centers; and

FIG. 4 is a pictorial representation of the approximate field distributions across the apertures of the transmit and receive horns.

Referring first to the plan view of FIG. 1 and the sectional view of FIG. 2 which respectively show the feed structure in the electrical and magnetic planes, the feed system 11 in accordance with the principles of the invention includes a transmit horn 1t) and a receive horn 12, both of which may be of the pyramidal type. The transmit and receive horns 10 and 12 have respective apertures 13 and 15 each aperture forming a flat plane. The transmit horn 1G is centered along a longitudinal axis 14 of the receive horn 12 in the dimensions of both the electrical and magnetic vectors of the respective FIGS. 1 and 2. The transmit horn 10 is joined with a waveguide section 13 which receives energy from a source of transmitting signals 2% The receive horn 12 is joined to a waveguide section 22. having an end wall 24. The transmit signal is at a substantially higher frequency than the received signal so that the dimensions of the waveguide sections 18 and 22 in the planes of both the electrical and magnetic vectors are selected to sustain a selected mode which may be the TE mode, for example. Generation of the fundamental mode in the receive horn 12 is provided by coaxial probes 26 and 28 which are coupled to a stripline hybrid ring 30 in a manner which excites the probes degrees out of phase to excite only the dominate TE mode in the Waveguide section 22. Types of hybrid rings that may be utilized for the hybrid 3%} are well known in the art and will not be explained in further detail. A received system 34 is coupled to the hybrid 30 for processing the received signal. It is to be noted that because of the isolation provided by the feed 11, a relatively small amount of microwave filtering, if any, is required at the receiving system 34.

Referring now also to the plan view of FIG. 3 which may be in the E plane, a reflector 38 which may have a parabolic surface configuration, is fixedly positioned relative to the feed 11 with a focal point FP located at a selected position within the flared portion of the transmit horn 12, and along the center axis 14. "It is to be noted that the reflector 38 is shown out of scale for convenience of illustration. The transmit horn It has a phase center PC and the receive born 12 has a phase center PC at respective distances 1, and 1 from the focal point FP along the center axis 14 and in the electrical plane. Similarly, as shown in FIG. 2, the transmit horn 1i and the receive horn 12 have respective phase centers PC and PC, at respective distances and from the focal point FP. It is to be noted that the distances Z l l and may result from deviations from a perfect design condition which may be desired to limit the time required to theoretically and experimentally design and select the parameters such as flare angles for a satisfactory condition of coincidence. However, it is to be understood that substantially perfect coincidence of phase centers in both the electrical and magnetic planes may be provided in accordance with the principles of the invention. Also, the deviations of the phase centers from absolute coincidence may be selected to provide a phase error no greater than the tolerance of the parabolic reflector, a greater coincidence being unnecessary for optimum system operation. The transmit horn 10 is projected a distance d in front of the aperture of the receive horn 12 to provide a selected high degree of isolation substantially without defocus in accordance with the principles of the invention. The flare angles 09 and are selected so that the phase centers PC and PC are substantially coincident with the focal point FP except for 1 and the small deviations in position that may be within the tolerances of the reflector system, for example, as provided principally by adjustments of the flare angles 0 and 0 In a similar manner, the flare angles 0 and 0 are selected in the H plane to provide substantial coincidence of the phase centers PC and PC; and desired proportioning of the lengths l and 1 As discussed above, the lengths l l l and 1 are made sufficiently small to be within the reflector or system tolerances. The distances and as well as the distances l and are selected to be substantially equal portions of the corresponding wavelengths so that essentially a focused condition is provided in the transmit and receive patterns.

To design the feed system in accordance with the invention,.aperture areas for the transmit and receive horns and 12 are selected so that a desired primary pattern is developed. The primary lobe pattern 42 shows the pattern configuration that may be developed by the transmit horn 10. The primary lobe pattern 43 shows the pattern configuration that may be developed by the receive horn 12. As is well known in the art, the pattern narrows or has a higher directivity for an increase of aperture dimensions. The focal length to diameter ratio F/D is known for the parabolic reflector 38. The width of the patterns such as 42 to properly illuminate the reflector 38 is governed by the relationship:

tan 2 4 f where is the half angle subtended -by the focal point and the edge of the reflector 38. Thus, the tapers of the patterns 42 and 43 are determined by selecting A and A and by selecting A and A The taper of the patterns 42 and 43 determines in the secondary pattern, the gain factor and the side lobe peak intensity below the level of the main beam and to a certain extent the beam width. The configurations of patterns 42- and 43 are selected principally as a function of the flare angle as explained in pages 359 to 363 of Microwave Antenna Theory and Design by Silver, in volume 12 of The Radiation Laboratory Series. Pattern degradation due to changes of flare angle is relatively small in the flare angle range of 10-20 degrees. Each pattern such as 42 and 43 is determined in both the E plane and in the H plane substantially independent of each other. During selection of the flare angles, the overall dimensions or packaging requirements may also be conidered.

The phase centers of the horns 10 and 12 considered independently of each other are known in the art to be a short distance within the aperture surface, the exact point being conventionally determined by experimental design. Conventional feed systems maintain the surface planes of the transmit and receive apertures coincident with each other to prevent loss of power in the secondary lobe because of phase error considerations. In the design procedure of the system of the invention, the phase centers PC and PC are selected to move forward toward the aperture by decreasing 6 to a value less than conventionally utilized, in order to be substantially coincident with respective phase centers PC and PC having positions determined by the projection distance d of the transmit horn 10. Increasing the flare angles of a horn in either the E or H planes characteristically moves the center of phase back toward the throat of the horn. The distance d is selected to provide a desired degree of isolation or filtering. The degree of coupling between the horns 10 and 12, is principally a function of the characteristics of the larger receive horn 12 and the pattern of the smaller transmit horn 10 as may be seen by the patterns shown on pages 359363 of the above referenced volume 12 of The Radia: tion Laboratory Series. Another factor that controls the phase center of the receive horn 12 is a loading effect of the transmit horn 10 which has been found to move the phase centers PC and PO; forward toward the aperture. As may be seen in FIG. 4, the inclusion of the transmit horn It in the receive horn 12 results in a central concentration of energy in both the E and H planes as shown by respective voltage and magnetic field amplitude curves 48 and 50. Also, it is believed that the projected outer surface of the transmit horn 10 conducts current patterns which are in front (in the direction of the reflector 38) of the aperture plane of the receive horn 12'. As a result, the concentration of electrical and magnetic vectors projects out in front of the receive aperture 15 as respectively shown by E vectors 29 and 31 and H vectors 33 and 35. The phase center PC has been found to be at a position forward from the position which would be expected Without the transmit horn 10 included therein. The phase centers are the center point of the positions of similar phase of the projected waves as indicated by arcs 47 of FIG. 3.

The phase centers PC and PC are thus selected for a relatively large projection distance d to be substantially coincident with each other except for the distance (l +l when design tolerances do not require absolute coincidence. A similar substantial coincidence is selected for the phase centers PC and PC; by proper adjustments of 0 and 0 The feed structure 11 is then positioned relative to the reflector 38 so that the focal point FP is on the center axis 14 and at a selected position between the phase centers PC and PC and between the phase centers PC and P0,. The lengths l and 1 as well as the lengths l and 1 are selected so that they are equal to a predetermined factor times the respective transmit and receive wavelengths A and A utilized in the system. Thus, the system of the invention provides essentially coincident phase centers but when a small amount of phase error is allowed by the system, the phase error may be equally distributed between the transmit and receive lobe patterns to provide a minimum system power loss.

During the above design procedure of the angles 0 and 0 to move the phase center PC forward toward coincidence with PC and to move the phase center PG, forward toward PC the calculated area of the receive horn 12 must be increased slightly by increasing the length C to provide a wider pattern 43. However, the above discused loading effect of the transmit horn 10 has been found in some feed arrangements in accordance with the invention to substantially cancel or minimize the required decrease of 0 during the design procedure so that the aperture dimensions in both the electrical and magnetic planes of the receive horn 12 are substantially unchanged as a result of providing approximately coincident phase centers. Thus it has been found that for some horn configurations and degrees of isolation of a selected projection distance d, the phase center PC is moved sufliciently toward the aperture by the loading eflect of the transmit horn 10 to provide the desired coincidence of phase centers without substantially decreasing the flare angle 6 from that determined by the above design procedure. It has also been found that in the system of the invention, the flare angles are sufliciently small that very small phase distortion is provided at the throats of the transmit and receive horns.

After the feed 11 and reflector 38 are mounted in the relative positions so that the focal point PP is in the proper position, the system operates with the desired secondary lobe pattern (not shown). The hybrid 30 having a 180 degrees phase shift and the dual probe balanced feeding arrangement cancels out undesired higher modes so that the energy is excited in the waveguide section 22 substantially as shown by the electric vectors of FIG. 4. The balanced feed allows operation in the TE mode without a metal partition separating the waveguide section into two portions in the electrical vector dimension.

As an illustrative example of one feed and antenna arrangement in accordance with the principles of the invention, the transmit born 10 may operate in the 7361 to 7363 megacycle per second bandwidth and the receive horn 12 may operate in the 1813 to 1822 megacycle per second bandwidth such as in an FM (frequency modulation) communication system. In the illustrative example, the reflector 38 has an F/D ratio of 0.4. The primary transmit and receive lobes 42 and 43 have tapers at 15 db (decibels) down in amplitude of 128 angular degrees in both the electrical and the magnetic plane as determined principally by the aperture dimensions. The following dimensions were utilized in the illustrative example:

A =1.37 in. d= in.

A :5.125 in. l =approximately 0.03 in. A =1.8O in. l =approximately 0.12 in. A =8.375 in. Z =approximately 0.03 in. b =1.37 in. l =approximately 0.12 in. [2 1.30 in.

b =137 in.

C123 in- C2=6 in.

C =3 in.

The transmit and receive portions of the feed operated in the TE modes. The balanced feeding arrangement is substantially as shown in FIG. 1. It is to be noted that the lengths l l l and L, are approximately the one-fortieth of the corresponding wavelengths.

In the system having the dimensions and frequency of operation of this illustrative example, over 100 db of filtering or attenuation of the transmit signal has been measured in the receiver as a result of the projection of the transmit horn 10. It is believed that substantially more attenuation is provided than this measured value, but test equipment limitations prevented measurement of larger attenuations at this time. The balanced feeding arrangement provided several additional decibels of isolation. Thus, a filter in addition to that provided by the feed system in accordance with the invention is normally not required. For a system requiring a greater isolation than provided by the improved feed, a simple filter will suflice at the receiver. In the system of the illustrative example, the phase error was maintained at a low value of less than 5 angular degrees with substantially the same phase error for both the transmit and receive lobes, The phase error developed at the throats of the horns 10 and 12 was maintained at a minimum. The secondary lobe pattern has side lobes which have a relatively small amplitude compared to the amplitude of the main lobe.

It is to be noted that although the system of the invention has been illustrated utilizing a circular parabolic reflector and a pyramidal rectangular horn element, the principles in accordance with this invention are also applicable for operation with parabolic reflector surface in only one dimension and sectoral horns in either the H or E planes. Also, the high isolation feed of the invention may be utilized for transmitting and receiving without a reflector in accordance with the principles of the invention.

Thus the antenna feed system in accordance with the principles of the invention, operating with transmit and receive signals in different frequency bands provides a high degree of isolation between the two apertures by projecting the inner horn a selected distance in front of the aperture plane of the outer horn. The composite feed has the signal phase centers substantially coincident for both the inner and outer horns so that this isolation or filtering is provided substantially without defocus of the secondary lobe patterns. Thus, the secondary pattern characteristics are optimized by the system of the invention. Therefore, the system of the invention provides a high degree of isolation between the transmit and received terminals and yet maintains highly desirable antenna pattern characteristics. The feed structure in accordance with the invention is relatively compact and simple to fabricate. The system of the invention provides high degree of isolation or filtering substantially without loss.

What is claimed is:

1. An antenna feed for transmitting energy and receiving energy in conjunction with a parabolic reflector having a center axis and a focal point, the transmitted energy being at a frequency substantially higher than the received energy comprising an inner and an outer pyramidal horn for respectively maintaining transmit and receive energy each having a center axis substantially coincident with the center axis of the reflector and each having a throat,- an aperture and selected flare angles, the aperture of said inner horn positioned a selected distance toward said reflector in front of the aperture of the outer horn to provide isolation in said outer horn from energy in said inner horn,

the flare angles of said inner and outer horns selected so that the phase centers of the transmitted and received energy are substantially coincident with the focal point.

2. An antenna feed structure for transferring energy of first and second frequencies, said second frequency being higher than said first frequency comprising a first pyramidal horn for transferring energy at said first frequency and having a longitudinal axis and having selected flare angles in both the electrical and magnetic planes to form an aperture,

and a second pyramidal horn for transferring energy at said second frequency and having a longitudinal axis substantially coincident with the longitudinal axis of said first horn and having dimensions so as to be positioned within said first horn, said second horn having selected flare angles to form an aperture, said second horn positioned so that the aperture thereof is a predetermined distance from the aperture of said first horn in the direction along said longitudinal axes of increasing horn size to provide isolation from energy being transferred between said second horn and said first horn.

3. An antenna feed for transmitting energy to and receiving energy from a parabolic reflector having a center axis and a focal point, the transmitted energy being at a frequency substantially higher than the received energy comprising an inner and an outer pyramidal born for respectively maintaining transmit and receive energy each having a center axis substantially coincident with the center axis of the reflector and each having a throat, an aperture and selected flare angles, the aperture of said inner horn positioned a selected distance toward said reflector in front of the aperture of the outer horn to provide isolation in said outer horn from energy in said inner horn,

the flare angles of said inner and outer horns selected so that the phase centers of the transmitted and received energy are on opposite sides of the focal point along said center axis respectively in directions toward and away from said reflector with the 3,325,817 7 8 distances from said focal point being a predetermined first microwave frequency and for receiving energy at a proportion of the wavelengths of the respective transsecond microwave frequency lower than said first fremitted and received energy. 'quency comprising 4. An antenna system for transmitting energy at a first a pyramidal transmit horn having a longitudinal axis microwave frequency and for receiving energy at a secand having throat dimensions for supporting energy 0nd microwave frequency lower than said first frequency "at the first frequency, said transmit horn having secomprising lected flare angles in both the electrlcal and maga pyramidal transmit horn having a longitudinal axis and having throat dimensions for supporting energy netic planes to form an aperture for radiating energy therefrom,

at the first frequency, said transmit horn having se- 10 a transmit waveguide coupled to the throat of said lected flare angles in both the electrical and magtransmit horn,

netic planes to form an aperture for radiating energy a pyramidal receive horn having a longitudinal ax1s therefrom, substantially coincident with the longitudinal axis and a pyramidal receive horn having a longitudinal of said transmit horn and having throat dimensions axis substantially coincident with the longitudinal f r Supporting energy at the second frequency, said axis of said transmit hor nd havin throat dirne transmit horn having the throat thereof positioned sions for supporting energy at the second frequency, within said receive horn, said receive horn having said transmit horn having the throat thereof posiselected flare angles in both the electrical and magtioned within said receive horn, said receive horn netic planes to form an aperture for receiving enhaving selected flare angles in both the electrical and ergy, the aperture of said transmit horn being posirnagnetic planes to form an a erture for re eiving tioned a selected distance forward from the aperenergy, the aperture of said t a it h b i ture of said receive horn along the, longitudinal axis positioned a selected distance forward from the aperin the direction of flare, the flare angles of Said ture of said receive horn along the longitudinal axis ei e o n cted so that the phase center of rein the direction of flare to provide substantial isola- Ceived gy is Substantially coincident With the tion in said receive horn from energy in said trans- Phase Center of energy transmitted from Said smit horn, the flare angles of said receive horn mit horn, a selected 50 that the phase center f i d a receive waveguide coupled to the throat of said reergy is substantially coincident with the phase cen- Ceive 110111, ter of energy transmitted fro aid t it h '30 and a balanced feed including first and second probes 5. An antenna system for transmitting microwave enpositioned in said receive waveguide and a hybrid coupled to said first and second probes. 7. An antenna feed for transmitting energy to and receiving energy from a curved reflector having a center axis and a focal point, the transmitted energy being at a frequency higher than the received energy comprising an inner and an outer pyramidal horn for respectively maintaining transmit and receive energy, each horn having a center axis substantially coincident with 40 the center axis of the reflector and each having a throat, an aperture and selected flare angles, the aperture of said inner horn positioned a selected distance toward said reflector in front of the aperture of the outer horn to provide isolation in said outer horn from energy in said inner horn,

the flare angles of said inner and outer horns selected so that the phase centers of the transmitted and received energy are substantially coincident with said ergy at a first frequency and receiving microwave energy at a second lower frequency comprising a parabolic reflector having a center axis and a focal point,

a transmit horn having a longitudinal axis substantially coincident with said center axis, having flared sides with selected flare angles and having an aperture and a throat, said throat having dimensions for maintaining energy at the first frequency,

a receive horn having a longitudinal axis substantially coincident with said center axis, having flared sides with selected flare angles and having an aperture and a throat, said throat having dimensions for maintaining energy at the second frequency, a portion of said transmit horn positioned within the aperture of said receive horn, the aperture of said transmit horn being positioned a selected distance in front of the aperture of said receive horn in the direction focalpoimalong said center axis from the throats to theaper- R f r Cited tures of said born to provide substantial isolation in said receive :horn from energy in said transmit UNITED STATES PATENTS horn, 2,425,488 8/ 1947 Peterson et al 34377 6 and with the flare angles of said transmit horn and 2,677,055 4/1954 Allen 343-776" said receive horn selected so that the phase centers 2,834,960 5/ 1958 Henderson 343-786 of the transmitted and received energy are substan- 2 972,148 2/1961 Rupp et a1 343 776 tially coincident with each other and with aid focal 3,109,996 11/1963 Allen 343 799 X point.

6. An antenna system for transmitting energy at a ELI LIEBERMAN, Primary Examiner. 

7. AN ANTENNA FEED FOR TRANSMITTING ENERGY TO AND RECEIVING ENERGY FROM A CURVED REFLECTOR HAVING A CENTER AXIS AND A FOCAL POINT, THE TRANSMITTED ENERGY BEING AT A FREQUENCY HIGHER THAN THE RECEIVED ENERGY COMPRISING AN INNER AND AN OUTER PYRAMIDAL HORN FOR RESPECTIVELY MAINTAINING TRANSMIT AND RECEIVE ENERGY, EACH HORN HAVING A CENTER AXIS SUBSTANTIALLY COINCIDENT WITH THE CENTER AXIS OF THE REFLECTOR AND EACH HAVING A THROAT, AN APERTURE AND SELECTED FLARE ANGLES, THE APERTURE OF SAID INNER HORN POSITIONED A SELECTED DISTANCE TOWARD SAID REFLECTOR IN FRONT OF THE APERTURE OF THE OUTER HORN TO PROVIDE ISOLATION IN SAID OUTER HORN FROM ENERGY IN SAID INNER HORN, THE FLARE ANGLES OF SAID INNER AND OUTER HORNS SELECTED SO THAT THE PHASE CENTERS OF THE TRANSMITTED AND RECEIVED ENERGY ARE SUBSTANTIALLY COINCIDENT WITH SAID FOCAL POINT. 